PLATYCRINID^. 717 



Platycrinus subspinosus Hall. 

 Plate LXVI. Figs. 9a, b. 



1858. Hai.l; Geol. Kep. Iowa, Vol. I., Part II., p. 536, Plate 8, Figs. 9 and 10. 

 1866. JIeek and Woethen; Geol. Rep. Illiuois, Vol, II., p. 173, Plate 15, Fig. 6. 



Smaller than P. discoideus; its dorsal cup has a similar form, but the 

 .surface of the plates is smooth or very obscurely corrugated ; the ventral 

 disk shorter, convex instead of pyramidal, and composed of but few large 

 plates ; the arms heavier, shorter, and without strias. 



Basal disk rather small, broadly and deeply funnel-shaped, the excava- 

 tions extending almost to the outer margins. The interbasal sutures very 

 slightly grooved, but the basi-radial and interradial sutures deep, and the 

 edges of the plates strongly beveled. Radials about once and a half as wide 

 as long, and nearly twice as wide at the upper end as at the lower, rising 

 very gradually to the facets, which project distinctly outward ; the limbs 

 curving rapidly upward and slightly inward. The facets, which are not 

 thickened around the edges, are directed horizontally, and their faces are 

 perfectly flat; they are wider than high and rather large, occupying nearly 

 two thirds tlie width of the plates. Costals very large for the genus, and 

 comparatively long ; pentagonal ; directed outward and slightly downward, 

 so that the succeeding distichals are sometimes at a lower level in the speci- 

 men than either radials or basals ; the upper angle quite obtuse. Distichals 

 two ; the first quadrangular ; the second pentangular ; but httle smaller 

 than the costals, giving off an arm to the outer side of the ray, and two 

 palmars to the inner side ; the latter supporting an arm to the inner side, 

 and two post-palmars to the outer, there being normally' four arms to each 

 main division and eight to the ray, exceptionally six or seven. Arms heavy, 

 moderately short, rounded on the back; composed of rather long, slightly 

 convex pieces. They are generally outstretched to about half their length, 

 then curve rapidly inward until their tips rest upon the outer margins of 

 the tegmen. Disk decidedly quinquelobate, higher than the dorsal cup ; 

 the ambulacra highly elevated into ridges. Orals very large, subspinous, 

 occupying one half of the ventral surface ; rather regular in their arrange- 

 ment. Disk ambulacra short, composed of but few tumid pieces. The inter- 

 ambulacral spaces small, sometimes formed of a single I'ow of three plates, of 

 which the middle one is quite large, the outer ones as long but narrower. 



